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47 lines
1.2 KiB
47 lines
1.2 KiB
=begin
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File: build_tree.rb
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Created Time: 2024-05-13
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Author: Xuan Khoa Tu Nguyen (ngxktuzkai2000@gmail.com)
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=end
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require_relative '../utils/tree_node'
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require_relative '../utils/print_util'
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### 構建二元樹：分治 ###
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def dfs(preorder, inorder_map, i, l, r)
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# 子樹區間為空時終止
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return if r - l < 0
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# 初始化根節點
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root = TreeNode.new(preorder[i])
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# 查詢 m ，從而劃分左右子樹
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m = inorder_map[preorder[i]]
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# 子問題：構建左子樹
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root.left = dfs(preorder, inorder_map, i + 1, l, m - 1)
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# 子問題：構建右子樹
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root.right = dfs(preorder, inorder_map, i + 1 + m - l, m + 1, r)
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# 返回根節點
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root
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end
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### 構建二元樹 ###
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def build_tree(preorder, inorder)
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# 初始化雜湊表，儲存 inorder 元素到索引的對映
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inorder_map = {}
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inorder.each_with_index { |val, i| inorder_map[val] = i }
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dfs(preorder, inorder_map, 0, 0, inorder.length - 1)
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end
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### Driver Code ###
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if __FILE__ == $0
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preorder = [3, 9, 2, 1, 7]
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inorder = [9, 3, 1, 2, 7]
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puts "前序走訪 = #{preorder}"
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puts "中序走訪 = #{inorder}"
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root = build_tree(preorder, inorder)
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puts "構建的二元樹為："
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print_tree(root)
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end
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